1. Field of the Invention
The present invention relates to a semiconductor device including a metal oxide semiconductor (MOS) transistor. In particular, the present invention relates to a semiconductor device including an n-type MOS (NMOS) transistor isolated by a shallow trench structure as an electrostatic discharge (hereinafter, referred to as ESD) protective element.
2. Description of the Related Art
In a semiconductor device including MOS transistors, an off transistor, which is an NMOS transistor provided in an off-state whose gate potential is fixed to a ground (Vss), is used as an ESD protection element for preventing breakdown of an internal circuit due to static electricity supplied from a pad provided for external connection.
Since the off transistor must flow a large amount of current generated by static electricity at once unlike ordinary MOS transistors forming an internal circuit such as a logic circuit, a large width (width W) of about several hundred micrometers is required for the transistor in many cases.
Though the gate potential of the off transistor is fixed to Vss to hold the off transistor in an off-state, the threshold voltage is less than 1 V as in the NMOS transistors constituting the internal circuit, permitting generation of subthreshold current to some extent. The width W of the off transistor is large as described above, and thus an off-leak current at standby during operation becomes larger, which leads to a problem of increase in the current consumption at standby during operation of the entire integrated circuit (IC) carrying the off transistor.
In particular, in the case of a semiconductor device in which a shallow trench is used for device isolation, there is a problem, in that an area adjacent to the shallow trench includes a region such as a crystal defect layer or the like which easily generates leak current, arising from the structure itself or a manufacturing method thereof, and hence it is difficult to reduce the off-leak current of the off transistor.
As a measure to reduce the leak current of the protection element, it is proposed to provide a plurality of transistors between the power line (Vdd) and the ground (Vss) so as to completely cut the current path therebetween (for example, see FIG. 1 of JP 2002-231886 A).
However, when the width W is made small to reduce the off-leak current of the off transistor, protection function cannot be sufficiently implemented. Besides, in a semiconductor device in which a plurality of transistors are provided to cut the current path between the power line (Vdd) and the ground (Vss) as proposed in JP 2002-231886 A, an occupation area thereof increases because the semiconductor device includes the plurality of transistors, leading to an increase in cost for the semiconductor device.